Abstract BK potassium channels play important roles in physiological and disease processes, and are attractive drug targets for the treatment of a wide variety of human disorders throughout the body. Compounds that inhibit or activate BK channels (modulators) have been proposed to treat stroke, epilepsy, psychoses, pain, traumatic brain injury, hypokaleamic periodic paralysis, asthma, COPD, angina, arterial hypertension, ischemic heart disease, erectile dysfunction, urinary incontinence, gastric hypermotility, irritable bowel syndrome, pancreatitis, cancer (brain, bone, and prostate), rheumatoid arthritis, glaucoma and diabetes. However, a challenge in realizing this therapeutic potential is that most BK channel modulators lack tissue specificity because the pore forming ?-subunit Slo1, which underlies most functional properties of the channel, is encoded by a single gene in all tissues. Our long-term goal is to develop potent tissue-specific modulators of human BK channels. Such compounds should induce little to no adverse effects in non-target tissue, and therefore could serve as effective therapeutics as well as important research tools for the study of BK channel function in selected organs. Properties in a modulator that are desirable in terms of tissue specificity include a dependence on accessory subunits that are expressed in a tissue dependent manner or a dependence on membrane voltage and intracellular calcium (Ca2+), since BK channels are activated by different combinations of these stimuli in different tissues. Using a novel high-throughput screen designed for the purpose we have identified nobiletin as a BK channel inhibitor with both of the above properties and a unique mechanism of action. Nobiletin inhibits BK channels composed of Slo1 alone or with the ?1 accessory subunit, in a voltage- and Ca2+- dependent manner, but fails to inhibit channels containing ?2 subunits. Moreover, nobiletin has known therapeutic effects on animal models of dementia, cancer and arthritis; but its primary molecular targets are poorly understood and it has never been identified as a BK channel modulator. The goals of the proposed research are to determine the state-dependence, subunit-selectivity, and mechanism of action of nobiletin. Accomplishing these goals will help define the potential tissue-specificity of nobiletin action and provide new insight into the mechanism and therapeutic action of this compound. Knowledge obtained from this study will also facilitate the design of future tissue-selective BK channel modulators. .